1. Field of the Invention
The present invention relates to an optical scanning device loaded in an electrophotographic image forming apparatus or the like.
2. Description of the Related Art
An electrophotographic image forming apparatus such as a digital copier, a laser printer or the like has an optical scanning device which deflects a light beam from a laser light source in a main scanning direction through a deflector, and forms a beam spot on a surface of a photosensitive body (surface to be scanned) moving in a sub-scanning direction (perpendicular to the main scanning direction). Thus, the optical scanning device forms a latent image of an image to be output on the surface of the photosensitive body.
As the deflector of the optical scanning device, a mechanical deflector employing a polygon mirror, a galvano-mirror or the like is used in general.
FIG. 1 shows an example of configuration of an optical scanning device in the related art.
The optical scanning device 40 causes a light beam 42 emitted from a laser light source 41 to be incident on a polygon mirror 47 rotating at high speed through a first optical system 46 consisting of a coupling lens 43, a diaphragm 44 and a line-image forming optical system 45, and causes the light beam to be reflected by a mirror surface 47a of the polygon mirror 47, the angle of which surface changes with the rotation of the polygon mirror 47. Thereby, the light beam 42 is deflected in the main scanning direction, and thus performs scanning repeatedly.
The light beam 48 reflected by the polygon mirror 47 passes through a second optical system consisting of an fθ lens 49, a long-dimension lens 50 and a turning mirror 51, and is used to scan a surface 53 a (surface to be scanned) of a photosensitive body belt 53 moving in the sub-scanning direction.
At this time, the laser light source 41 is intensity-modulated according to an image to be output, and, thereby, an electrostatic latent image is formed on the surface 53a of the photosensitive body belt 53 in a form of a dot pattern by the blinking light beam 48.
As mentioned above, a mechanical deflector employing a polygon mirror or a galvano-mirror is used as a deflector of an optical scanning device in general. Because each of the polygon mirror and galvano-mirror is driven by a motor, heat is generated by the motor when it is driving the mirror.
Recently, as image output speed is increased, the driving speed of a deflector of an optical scanning device is increased. Thereby, the amount of heat amount generated by the motor of the deflector increases. Accordingly, it becomes to ignore the influence thereof on other optical components (a light source, a coupling lens, a diaphragm, a line-image forming optical system, an fθ lens, a long-dimension lens, and so forth) contained in a housing of the optical scanning device together with the deflector.
Heat generated by the motor is transmitted to the respective optical components mainly through the housing. Thereby, the respective optical components are heated, and, accordingly, problems may occur such as a shift of image position (focus shift), thickening of light beam, error of writing magnification, and so forth. As a result, image quality may be degraded.
Further, as the driving speed of a deflector is increased, a vibration generated thereby when the deflector is driven capable of being ignored. Accordingly, it has become impossible to use a plastic as a housing of an optical scanning device, and a metal housing has now therefor been employed. Because metal has a heat conductivity larger than that of plastic, heat from a motor of a deflector comes to be more easily transmitted by the housing. As a result, other respective optical components come to be affected by the heat from the motor of the deflector more seriously. Accordingly, the above-mentioned problems occur more remarkably.
Further, recently, lenses and/or mirrors which are main components of optical systems of an optical scanning device have been made of plastic, and are made to have spherical surfaces so that both high performance and a low cost of the device has resulted directed.
However, because plastic-made components are not superior in heat-resistant property in comparison with glass-made components, lenses and mirrors which are main components of optical systems are more seriously affected by heat from a motor.
Accordingly, the above-mentioned problems have occurred more often.
In the related art, Japanese Laid-Open Patent Application No. 7-244249 discloses an idea for avoiding influence of heat from a deflector.
According to Japanese Laid-Open Patent Application No. 7-244249, a scanning lens is separated from other optical components (mainly a deflector) in a housing, and, thereby, shifting of the scanning position and a change of diameter of light beam occurring due to shake of air surrounding the scanning lens are prevented. This technique is directed to eliminate the influence of air disturbances (air flow generated by the deflector) around the deflector due to rotation of a mirror of the deflector, basically.
However, according to this idea, although the deflector is separated from optical components such as the scanning lens, a turning mirror and so forth which direct light reflected by the deflector to a surface to be scanned, the deflector is not separated from other optical components such as a coupling lens, a diaphragm, a line-image forming optical system which direct a light beam to the deflector. Accordingly, those other optical components are affected by heat generated by the deflector.
As a result, in the housing, some optical components are affected by heat from the deflector, and the other optical components are not. Generally speaking, in order to prevent the characteristics of respective optical components included in a optical system from shifting unevenly, the design is such that the entirety of the optical system is in a uniform temperature environment, and, when the temperature environment changes, the system can be corrected as a whole. Accordingly, when some optical components are in a different temperature environment, a design for correction with respect to environment change becomes difficult to obtain.